Seismic system with ghost and motion rejection

ABSTRACT

An underwater seismic system for reducing noise due to ghost reflections or motion through the water from seismic signals. The system includes two motion sensors. One sensor has a first response and is sensitive to platform-motion-induced noise as well as to acoustic waves. The other sensor has a different construction that isolates it from the acoustic waves so that its response is mainly to motion noise. The outputs of the two sensor responses are combined to remove the effects of motion noise. When further combined with a hydrophone signal, noise due to ghost reflections is reduced.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 13/011,358, filed Jan. 21, 2011, which claims priority to U.S.Provisional Patent Application No. 61/297,656, filed Jan. 22, 2010. Bothprior applications are incorporated entirely by reference into thisspecification.

BACKGROUND

The invention relates generally to marine seismic prospecting and inparticular to apparatus and methods for reducing the effects ofundesired seismic reflections and noise in sensors towed behind a surveyvessel, in sensors laid on the sea bottom, or in sensors in autonomousnodes.

In towed marine seismic exploration, a hydrophone array is towed behinda marine vessel 20 near the sea surface 22, as in FIG. 1. Thehydrophones are mounted in multiple sensor cables commonly referred toas streamers 24. The streamers serve as platforms for the hydrophones. Aseismic sound source 26, also towed near the sea surface, periodicallyemits acoustic energy. This acoustic energy travels downward through thesea, reflects off underlying structures or subsea strata 28, and returnsupward through the sea to the hydrophone array. Reflected seismic energyarrives at towed-array receive points. The hydrophone array containsmany such receive points and records the upward traveling seismicacoustic wavelet from the seabed 30 at each of the receive points. Thehydrophone recordings are later processed into seismic images of theunderlying structures.

Noise is a major consideration in towed streamer operations. Noisesources include swell noise and wave noise from the sea surface. Andtowing the streamer through the water causes noise. Some of this noisepropagates through the streamer and some through the water columnitself. The typical way of dealing with noise sources is to use acombination of temporal and spatial filtering. Temporal filtering isaccomplished by discrete digital sampling of the hydrophone signals intime with weighting applied to the samples. The hydrophone channels alsoinclude analog filters to prevent aliasing of signals at frequenciesgreater than half the sample rate. The spatial samples are typicallyformed by group-summing individual hydrophone outputs so that pressurenoise propagating along the length of the streamer is attenuated. Thisspatial sampling has no impact on noise that propagates in a directionorthogonal to the streamer axis. Typical hydrophone groups consist ofeight or so hydrophones in a 12 m section of the streamer.

Acoustic impedance, pc, is the product of the density and the speed ofsound in a medium. Reflection of at least some of the sound-wave energyoccurs whenever a change in acoustic impedance is encountered by thesound waves. The energy that is not reflected is transmitted (refracted)beyond the boundary between the two regions of different acousticimpedances. The pressure waves are compression waves, which induceparticle motion in the direction of propagation. At a planar interfacebetween two different homogenous media, a sound wave reflects at anangle equal to the angle of incidence θ₁ and refracts at an angle θ₂.The refraction angle is given by:

θ₂=sin⁻¹(c ₂ sin θ₁ /c ₁).

The subscript refers to the sound wave moving from medium 1 to medium 2and c₁ and c₂ are the speeds of sound in each medium. If the incidentangle θ₁ is zero, then the refracted energy propagation path will be atan angle θ₂ of zero.

For an incident angle θ₁ of zero and no energy converted to shearenergy, the reflection coefficient at the water-air interface isdescribed by:

$\begin{matrix}{R_{pp} = \frac{{\rho_{2}c_{2}} - {\rho_{1}c_{1}}}{{\rho_{2}c_{2}} + {\rho_{1}c_{1}}}} \\{\approx {- 1.}}\end{matrix}$

The reflected energy at the water-air interface is R² _(pp), or nearly1, making the sea surface a near perfect reflector of sound energy.After returning from the sea bottom or the target of interest, theenergy is again reflected by the sea surface back to the streamer.Because a typical hydrophone has an omni-directional response, thehydrophone array also records a ghost response, which is the seismicacoustic wavelet reflected from the sea surface and arriving delayed intime and reversed in polarity. The ghost is a downward-traveling seismicacoustic wave that, when added to the desired wave, detracts from therecorded seismic image. The ghost-causing reflection can also continueto the sea bottom or other strong reflector and be reflected back up toagain interfere with the desired reflections and further degrade theimage. These reflections are commonly referred to as multiples.

For a vertically traveling pressure wave, the ghost produces a notch inthe frequency spectrum of a hydrophone response at f_(notch)=c/2d, wherec is the speed of sound and d is the streamer depth. Seismic streamershave been conventionally towed at a depth of 10 m or less. At a depth of10 m, the notch frequency (f_(notch)) is 75 Hz. A frequency responseextending beyond 100 Hz is required for high seismic image resolution.Because the notch frequency is inversely proportional to the tow depth,streamers are often towed at shallower depths to improve the resolutionof a seismic image. Towing at shallow depths is problematic becausenoise from the sea surface begins to interfere with the desired seismicsignals. These effects are worsened as weather deteriorates, sometimescausing the crew to discontinue operations until the weather improves.The elimination of ghost-notch effects would enable towing at greaterdepths farther away from surface disturbances.

Ocean bottom systems, in which the seismic sensors are placed on theseabed, reject ghosts and multiples by a technique commonly known as p-zsummation. In an acoustic wave, the pressure p is a scalar, and theparticle velocity u is a vector. A hydrophone, with a positiveomni-directional response, records the seismic acoustic wave pressure p.A vertically oriented geophone or accelerometer records the verticalcomponent of the seismic acoustic-wave particle velocity u_(z), with apositive response to up-going signals and a negative response todown-going signals. In p-z summation, the velocity signal is scaled bythe acoustic impedance pc of seawater before it is added to the pressuresignal. A gimbaled single-axis sensor is also scaled to account for thechange in sensitivity of the particle-motion sensor due to the off-axisarrival of any received signals. If an accelerometer is used, its outputsignal can be integrated to obtain the velocity signal, or thehydrophone signal can be differentiated so that it can better spectrallymatch the accelerometer. This produces a compound sensor that has a fullresponse to the upward traveling wave and at least a partially mutedresponse to the downward traveling wave to reject the ghost andmultiples. One such method of signal conditioning and combination ofsignals to get a single de-ghosted trace is described in U.S. Pat. No.6,539,308 to Monk et al. FIG. 2 is a two-dimensional (2D) representationof the response of a particle-velocity sensor. FIG. 3 is a 2Drepresentation of the response of an omni-directional hydrophone summedwith the response of a vertical particle-motion sensor. The fullthree-dimensional responses can be envisioned by rotating the 2Dresponses about their vertical axes.

Recently there has been interest in using techniques similar to p-zsummation in towed-streamer acquisition to allow deeper tows withoutinterference from ghost-notch reflections. Operating a particle-motionsensor in a seismic streamer presents a problem because the streamerexperiences accelerations due to towing or sea surface effects that arelarge compared to accelerations caused by the desired seismicreflections. Moreover, these unwanted accelerations are in the samespectral band as the desired reflection response. When a towing vesselencounters ocean waves, there are small perturbations in the speed ofthe vessel. The vessel also typically undergoes a yawing motion. FIG. 4depicts energy being imparted to the streamers 24 by speed variations 32and yawing motion 34. FIG. 5 is a side view depicting energy causingaccelerations and transverse waves in the streamer 24. (The energy'seffect on the streamer is exaggerated in FIG. 5 for illustrativepurposes.) Most of the energy is attenuated by elastic stretch members36, typically in front of the sensing arrays. While the energy isgreatly attenuated, some does remain. Accelerations a caused by planarpressure waves due to the desired seismic reflections are given by:

${a = \frac{p\; 2\pi \; f}{Z}},$

where p=the acoustic sound pressure level, f is the frequency, and Z isthe acoustic impedance. Performance of a particle-velocity measuringsystem should be near the ambient noise limits. Typically, seismic-datacustomers require ambient noise from streamer hydrophone systems to bebelow 3 μbar. Since the acoustic impedance of seawater is 1.5 MPa·s/m, a3 μbar pressure wave at 4 Hz produces particle accelerations of roughly0.5 μg. FIG. 6 shows a mechanical model of the frequency response oftypical cable axial accelerations in the middle of a streamer. Thepresence of a secondary peak at 4 Hz, only 1.5 orders of magnitude belowthe primary peak, indicates that, in some cases, the cable dynamicmotion can be greater than the seismic signal to be measured.

U.S. Pat. No. 7,167,413 to Rouquette uses an accelerometer in a seismicstreamer to reject the ghost-notch effect. Rouquette uses a mass-springsystem to reduce the effect of cable dynamics on the accelerometer and aload-cell system to measure and reject the cable-motion-induced noise onthe accelerometer. The Rouquette system relies on well-known complexmechanical relationships that do not remain constant with manufacturingtolerances, aging, and environmental conditions. Rouquette uses asignal-processing adaptive algorithm to derive the relationship of theload-cell-sensor-and-mass-spring system to the acceleration acting onthe accelerometer in situ. Rouquette describes a complex mechanical andelectronic system.

U.S. Pat. No. 7,239,577 to Tenghamn et al. describes an apparatus andmethod for rejecting the ghost notch using an acoustic-waveparticle-velocity sensor. Tenghamn et al. teaches the use of afluid-damped, gimbaled geophone. It is known in the art that the fluidencapsulating the geophone is chosen to provide damping of the sensorswinging on its gimbals. While not described in Tenghamn et al., it isknown in the art that a mass-spring vibration-isolation system canreduce the effect of cable mechanical motion on the geophone response.Motion of the geophone caused by cable mechanical motion isindistinguishable from acoustic-wave particle motion in the geophoneresponse. The desired seismic-wave particle motion is obscured by cablemechanical motion in Tenghamn et al. This technique also gives theresponse similar to the cardioid in FIG. 3, where there are stillundesired signals coming from the surface and being induced by streamerexcitation along the streamer axis.

U.S. Pat. No. 7,359,283 to Vaage et al. describes a method of combiningpressure sensors and particle-motion sensors to address the impact ofmechanical motion on the particle-motion sensors. In this method, theresponse of the particle-motion sensor below a certain frequency f₀ isnot used, but only estimated from the pressure-sensor response and theknown pressure-sensor depth. The frequencies rejected are those forwhich mechanical motion of the streamer is expected. The estimatedresponse has poor signal-to-noise ratio at the lower frequencies ofinterest. This rejection below a certain frequency is not optimal as italso rejects valid signals in an important low-frequency band wheredeep-target data is likely to exist.

While these patents all describe methods to reject the ghost notch in aseismic streamer, none adequately addresses the effects of streamer towand other noise that affects the particle-motion sensor or hydrophonemeasurements. All also fall short of producing high-fidelity, sensedacoustic-wave components with good signal-to-noise ratio down to thelowest frequencies of interest.

SUMMARY

These shortcomings are addressed by an underwater seismic systemembodying features of the invention. Such a system comprises a firstmotion sensor that can be used on an underwater platform and has a firstresponse and a second motion sensor that is disposed proximate to thefirst motion sensor and has a second response. The first and secondresponses are similar in magnitude for platform motion and different foracoustic wave particle motion.

One version comprises a first motion sensor having a first acousticimpedance to produce a first sensor signal representing platform motionand acoustic waves and a second motion sensor disposed proximate to thefirst motion sensor and having a second acoustic impedance to produce asecond sensor signal representing platform motion and representingattenuated particle motion due to acoustic waves. Means for combiningthe first sensor signal and the second sensor signal attenuates noisedue to platform motion and produces a response to particle motion due toacoustic waves.

Yet another version comprises a first motion sensor and a second motionsensor disposed proximate to the first motion sensor. An acoustic shieldis arranged to shield only the second motion sensor from acoustic waveparticle motion.

BRIEF DESCRIPTION OF THE DRAWINGS

These aspects and features of the invention are better understood byreferring to the following description, appended claims, andaccompanying drawings, in which:

FIG. 1 is a side elevation view of a typical seismic survey operationshowing an array of hydrophones under tow behind a survey vessel anddepicting reflected seismic energy arriving at towed-array receivepoints;

FIG. 2 is a two-dimensional graph of the response of a particle-velocitysensor;

FIG. 3 is a two-dimensional graph of the response of an omni-directionalhydrophone summed with the response of a vertical particle-velocitysensor;

FIG. 4 is a top plan view of a typical survey as in FIG. 1 depictingtow-speed fluctuations and yaw;

FIG. 5 is a side elevation view of a survey as in FIG. 4 depicting theexaggerated effects of tow-speed fluctuations and yaw on streamer shape;

FIG. 6 is a plot of typical accelerations of a streamer in a survey asin FIG. 1;

FIG. 7 is a block diagram of a general version of an underwater seismicsystem embodying features of the invention including two motion sensorswith different acoustic responses;

FIG. 8 is a frequency-domain block diagram of the responses of motionsensors as in FIG. 7 to the acoustic-wave component of incident acousticenergy;

FIG. 9 is a frequency-domain block diagram of the responses of motionsensors as in FIG. 7 to the platform-motion component of incidentacoustic energy;

FIG. 10 is a time-domain plot of the output of a motion sensor as inFIG. 7 that is responsive to platform motion and acoustic (pressure)waves;

FIG. 11 is a time-domain plot of the output of a motion sensor as inFIG. 7 that is responsive only to platform motion;

FIG. 12 is a plot of the difference between the outputs of FIGS. 10 and11 representing an acoustic (pressure) wave signal with platform motionremoved;

FIG. 13 is one version of a seismic system as in FIG. 7 in which themotion sensors are housed in different structures, which providedifferent acoustic impedances;

FIGS. 14A and 14B are cross-sectional views of another seismic system asin FIG. 7 having multiple motion sensors axisymmetrically arranged in astreamer;

FIG. 15 is yet another version of a seismic system as in FIG. 7 in whicheach motion sensor has a different acoustic cross-section to providedifferent acoustic responses;

FIG. 16 is an alternative version of the seismic system of FIG. 15 withhigher gain;

FIG. 17 is a side elevation view of a seismic system as in FIG. 7mounted in cable-positioning birds rotatably suspended from a streamer;and

FIG. 18 is a side view of a seismic system as in FIG. 7 mounted in acable-positioning bird connected in line between streamer sections.

DETAILED DESCRIPTION

FIG. 7 is a block diagram of a general version of an underwater seismicsystem 19 embodying features of the invention, which comprisestechniques for using motion sensors, or sensor assemblies, withdifferent responses to sound-wave-induced signals and similar responsesto platform, e.g., streamer, cable, or autonomous node, motion toimprove the signal-to-noise ratio of data acquired for seismic imaging.In FIG. 7, two motion sensors 40, 41 and one pressure sensor 42,generally a hydrophone, provide signals that are combined to produce anoise-reduced and de-ghosted signal. A group of pressure sensors can beused in lieu of a single sensor, e.g., to reduce the noise arising frompressure waves propagating along the streamer axis. The motion sensorsideally are dc-sensitive and are capable of resolving the gravityvector; otherwise, an additional orientation sensor is used. The firstmotion sensor 40 has a response to acoustic waves that is ideally, butnot necessarily, equal to that of seawater; its response may beincreased beyond that of seawater if more gain is desired. The secondmotion sensor 41 has a response to acoustic waves that is measurablydifferent from that of the first motion sensor 40. This difference inacoustic response can be realized by means of a difference in thematerial composition or the geometric configuration of the sensors. Inall versions of the system, the material and geometric properties ofboth sensors are chosen so that their mechanical responses to platformmotion are matched. For example, if each motion sensor is designed tointeract with a cable in the same way as a second-order mass-springsystem, then the masses (including added mass, if appropriate) of thesensors and their associated spring constants are made equal. The firstand second outputs 44, 45 of the first and second motion sensors 40, 41are subtracted 46, either locally or after remote processing, to producea reduced-noise response signal 48 indicating particle motion due toacoustic waves with platform motion attenuated. The subtraction block 46constitutes one means for combining the first sensor signal and thesecond sensor signal. If the signal of one of the sensors is reversed inphase, the means for combining the first sensor signal and the secondsensor signal would be realized as an addition block instead. Thereduced-noise response is scaled 50 to match the pressure-sensorresponse 52, e.g., the hydrophone signal, and used in p-z summationmeans 54 to produce a final output signal 56 that also rejects ghostnotches and multiples. The means for combining the first sensor signaland the second sensor signal and the p-z summation means may be realizedlocally by analog circuitry, by digital logic circuitry, oralgorithmically in a microprocessor, remotely in a shipboard computer orin off-line data processing.

FIG. 8 is a block diagram of the two motion sensors 40, 41 of FIG. 7 inthe frequency domain indicating their transfer functions to the acousticwave component 58 of incident energy. The acoustic wave componentincludes the seismic signals of interest. The first sensor 40 and thesecond sensor 41 have unequal acoustic wave transfer functions H₁(s) andH₂(s). The transfer function H₁(s) is sensitive to acoustic waveparticle motion, so that the first sensor 40 produces an output responseO₁(s) that represents particle motion. The transfer function H₂(s) isinsensitive to acoustic wave particle motion, and the second sensor 41produces an output response O₂(s) that does not include the motion ofsurrounding acoustic-medium particles. FIG. 9 is a block diagram of thetwo motion sensors 40, 41 of FIG. 7 in the frequency domain indicatingtheir transfer functions to the platform-motion component 59 of incidentenergy. The transfer functions H₃(s) and H₄(s) of the two motion sensors40, 41 to platform motion are proportional (or equal) in magnitude, butcould be opposite in phase. Thus, both sensors 40, 41 have similaroutput responses O₃(s) and O₄(s) to platform motion. The compositetransfer functions of the first and second motion sensors 40, 41 toincident energy are the combinations of H₁(s) and H₃(s) for the firstsensor and of H₂(s) and H₄(s) for the second sensor. The compositeresponses of the two sensors are the combinations of O₁(s) and O₃(s) forthe first motion sensor and of O₂(s) and O₄(s) for the second motionsensor. FIG. 10 is an example representation of the time-domain responseof the first sensor 40 to incident energy that includes both platformmotion and acoustic waves. The first sensor's response 44 is sensitiveto both platform noise and the acoustic wave. FIG. 11 is thecorresponding response of the second sensor 41 to the same incidentenergy. The second sensor's response 45 is sensitive only to theplatform-noise component of the incident energy. FIG. 12 plots theresult of combining the responses of the two sensors by subtracting theoutput 45 of the second sensor from the output 44 of the first sensor toproduce the noise-subtracted acoustic wave signal 48 of FIG. 7.Although, for purposes of simplifying the description, the response ofthe second sensor to pressure waves was treated as zero, it may havesome slight response, or even a negative response, to pressure waves.Furthermore, the first and second sensor outputs may not be exactlymatched to streamer vibrations. But, even in these instances, the signalsubtraction still results in an acoustic wave response with a greatlyattenuated platform-motion response that can be scaled and combined withthe hydrophone data by p-z summation.

Various specific versions of the general system indicated in the blockdiagrams of FIGS. 7-9 use different levels of acoustic impedance toachieve the desired difference in response to acoustic wavelets. Asdescribed above, the two motion sensors 40, 41 and the pressure sensor42 are mounted in, on, or to a platform. For example, they may beenclosed in an underwater streamer or mounted inside a cable-positioningbird attached to a streamer. The motion sensors are isolatedacoustically from each other, but are located close together andseparated into individual regions by a divider, for instance. The firstmotion sensor is enclosed in a first region with an exterior whoseacoustic impedance is similar to that of the surrounding seawater sothat acoustic waves penetrate the exterior with minimal reflections andact on the sensor. The second motion sensor is located in anacoustically opaque enclosure in a second region and is not affected byincident acoustic waves. The streamer itself, being under tension, has asmall and erratic response to the acoustic waves. Any response of thestreamer itself to the acoustic waves is recorded as platform motion.Therefore, the first sensor has a proportional response to acousticwaves; and the second sensor has a negligible response. Additionally,the sensor assemblies are calibrated to have matched responses toplatform motions, (e.g., streamer vibrations), for instance by equatingtheir masses (including added mass, if appropriate) and associatedspring constants if they behave as second-order mass-spring systems.Subtraction, either locally or after remote processing, of the secondsensor signal from the first sensor signal accordingly yields thedesired acoustic wave signal with greatly attenuated streamer-motionresponse.

One specific version of the seismic system of FIGS. 7-9 is shown in FIG.13 with two motion sensors 60, 61—separated acoustically by a centraldivider 64—and a pressure sensor 62. The first motion sensor 60 iscontained in a first region 66 of the streamer with a rigid,acoustically transparent exterior 68. For example, the exterior 68 is aperforated, rigid housing covered with a flexible, acousticallytransparent skin 70. The interior of the first region 66 is filled withfluid. Ideally, the skin and fluid both have acoustic impedances equalto that of the surrounding seawater. A first test mass 72 with anacoustic response ideally, but not necessarily, equal to that of thefluid is suspended in the fluid; its response may be increased beyondthat of seawater if more gain is desired. The first test mass 72 isconnected to the exterior of the streamer by means of a displacement,velocity, or acceleration sensor, which serves as the motion sensor. Thefirst sensor 60 uses the exterior of the streamer as a frame ofreference and acts as a spring in coupling the test mass and streamerdynamically. The first sensor can be single crystal or a PZT bender, forinstance. If the sensor is a single-axis sensor, multiple test-masssystems can be used to form a tri-axis sensor, with all test massescalibrated to match in both acoustic and dynamic response. Analternative for multi-axis measurement is to connect several sensors toa common test mass for multi-axis measurement as long as the mass sensorresponses can be kept independent. The second sensor 61 and a secondtest mass 73 are connected in an assembly in a second region 67 on theopposite side of the divider from the first region 66. The secondsensor's assembly differs from the first sensor's in that its housingexterior 69 has an acoustic impedance much greater than that of thesurrounding seawater and its interior 67 is filled with air to accountfor any non-negligible elasticity in the housing exterior 69. Augmentingthe effects of the increased acoustic impedance of the second sensor'shousing is its rigidity, which allows the housing to act as an acousticshield, analogous to a Faraday cage in electromagnetism. The acousticimpedance of the second housing exterior 69 is set with a materialhaving a suitably high density or sound speed.

Another version of a seismic system embodying the invention is shown inFIGS. 14A and 14B with two sets 80, 81 of motion sensors and a pressuresensor 82. In this version, the first sensor set 80 and the secondsensor set 81 are connected to a single rigid body 84 that carriesstreamer vibrations. The rigid body has a large-diameter first portion86, a smaller-diameter second portion 87, and a transition section 88joining the first and second portions. The smaller-diameter portion 87is tubular in shape with an inner side 83 and an outer side 85. Thefirst sensor set 80 encircles a section of the second portion 87 of therigid body 84 and is connected to its outer side 85. Three or moreindividual sensors may be used to constitute the first set 80. Ifaxisymmetry is not employed, then the first sensor set 80 is insteadlocated alongside the rigid body. An acoustically transparent exterior90, which may consist of a flexible membrane over a perforated, rigidhousing, separates the sensor system from the surrounding seawater. Afirst cavity 92, between the second portion 87 of the rigid body 84 andthe exterior 90, is filled with fluid. Ideally, the exterior 90 and thefluid have acoustic impedances equal to the acoustic impedance of thesurrounding seawater. A first test mass 94, with acoustic propertieslike those of the first test mass in FIG. 13, is suspended in the firstcavity 92 and encircles the second portion 87 of the rigid body 84. Thefirst test mass 94 is coupled mechanically to the outer side 85 of therigid body 84 by the first set 80 of motion sensors with properties likethose of the first sensor 60 in the version of FIG. 13, but with therigid body 84 as their frame of reference. A second cavity 93 iscontained entirely within the tubular second portion 87 of the rigidbody 84. The second cavity 93 contains a second test mass 95 suspendedin fluid and coupled to the rigid body 84 by the second set 81 of motionsensors connected to the inner side 83 of the rigid body. The dynamicresponse of the second set 81 of sensors is calibrated to have aresponse to streamer vibrations that matches the response of the firstset 80. Unlike the first test mass 94, however, no requirements areplaced on the acoustic response of the second test mass 95. The rigidbody 84 itself acts as an acoustic shield to the second sensor set 81and is composed of a material with relatively high acoustic impedance. Abenefit of this coaxial arrangement is that multiple individual sensorsrespond to the accelerations of each test mass. Combining the outputsignals of the motion sensors leads to a more robust estimate of theactual acceleration values. As depicted, the first and second sensorsets 80, 81 are sensitive to radial motion; an additionaltest-mass-sensor system may be included in each cavity in alignment withthe streamer axis if tri-axis sensitivity is needed.

Yet another version of a seismic system is shown in FIG. 15. A streamerwith a rigid, acoustically transparent exterior 98 has two motionsensors 100, 101, such as dc-sensitive, tri-axis accelerometers, and onepressure sensor 102, such as a hydrophone. The exterior 98 may comprise,for instance, a perforated, rigid housing covered with a flexible,acoustically transparent skin. The accelerometers can be realized bymicroelectromechanical system (MEMS), PZT, single crystal, or any othertechnology with similar utility. The motion sensors 100, 101 are rigidlymounted to first and second rigid housings 104, 105 to enable directmeasurement of any dynamic streamer motion. Both sensors are coupledacoustically to the cable exterior 98, but are isolated acousticallyfrom each other, for instance, by a central divider 106. Each of thefirst and second housings 104, 105 is constructed such that the mass ofthe first housing plus the mass it encloses equals the mass of thesecond housing plus the mass it encloses. The dynamic couplings 103between the housings and the streamer exterior 98 are designed to act assecond-order mass-spring systems with equal spring constants so that theequality of the mass-spring relationships is preserved. On the otherhand, the housings have different acoustic cross-sections so that theygenerate different responses to acoustic pressure waves. Specifically,the first sensor 100 generates a first sensor signal 108 that is a goodrepresentation of the acoustic particle motion; the second sensor 101produces a second sensor signal 109 that is largely insensitive toacoustic waves. The sensor housings are constructed with differentgeometries, and possibly also with different materials, to effectdifferent cross-sections and, thus, different transfer functions foreach sensor. The second sensor signal 109 is subtracted 107 from thefirst sensor signal 108 either locally or after remote processing, toprovide the desired pressure wave signal with greatly attenuatedresponse to streamer motion. Open-cell foam can be used, for example, toserve as the dynamic coupling 103 between each housing 104, 105 and theexterior 98. Filled with a fluid calibrated to match the acousticimpedance of the surrounding seawater, the foam can serve also as atransparent acoustic coupling. In this example, the first housing 104 issealed with respect to the fluid and filled with air to account for anynon-negligible elasticity in the housing; and the second housing 105 isperforated or slotted and allowed to fill with the surrounding fluid.The resultant disparity in overall density between the housings accountsfor their different responses to incident pressure waves.

A modified version of the seismic system of FIG. 15 intended to enhancethe overall gain of the system is shown in FIG. 16. The first sensor 110behaves acoustically and dynamically like the first sensor 100 in FIG.15. The second sensor 111 produces a response to pressure waves thatmatches that of the first sensor 110 and a streamer-motion responseequal in magnitude but opposite in polarity to that of the first sensor.The first housing 114 and the second housing 115 are constructed as inFIG. 15, particularly in terms of acoustic cross-section and density, sothat they have a similar mass-spring response to cable motion, but ameasurably different response to incident acoustic pressure waves. Thesecond housing 115 additionally includes a test mass 116 that isdesigned to oscillate in a fluid and have an acoustic wave responsematching that of the first housing 114. On the other hand, the responseof the test mass to streamer motion is much less than that of thehousings because the test mass is suspended in a fluid and the housingsare coupled mechanically to the cable exterior. The test mass 116 isconnected non-rigidly to the second housing 115 by means of adisplacement, motion, or acceleration sensor 111 that uses the secondhousing as a frame of reference. In this example, a cantileveredaccelerometer, composed of piezoelectric materials, is used as themotion sensor. Multiple accelerometers can be employed to form atri-axis sensor, with each test mass calibrated to match the acousticresponse of the first housing 114 in its respective axis. Pressurewaves, which impart motion on the test mass 116, but not on the secondhousing 115, are therefore detected positively, i.e., in phase. Sopressure signals from the first sensor 110 and the second sensor 111match in both magnitude and sign. Conversely, streamer vibrations, whichinfluence the second housing 115, but not the test mass 116, aredetected negatively, i.e., opposite in phase. So vibration signals fromthe sensors match in magnitude, but have opposite signs. In this casethe signals from the two sensors 110, 111 are combined by addition 118,rather than subtraction, to produce a greatly diminished streamer-motionresponse and a simultaneous increase in gain of the acoustic waveresponse. Alternatively, another cantilevered test mass in the firsthousing 114 could be used. But, because the first sensor signal wouldalso be reversed in polarity, it would have to be combined with thesecond sensor signal by subtraction rather than addition.

As shown in FIG. 17, the sensor portion of the seismic system 19 can bemounted within a streamer cable 120 or within a cable-positioningdevice, such as a cable-leveling or cable-steering bird 122, rotatablyattached to the streamer by collars 124. As shown in FIG. 18, acable-positioning device 126 connected in line between fore and aftstreamer sections 128, 129 can house the sensor portion of the seismicsystem 19. Clearly, the sensors can be mounted in other devicesattachable in, on, or to a streamer, an ocean-bottom cable, or anautonomous node.

A tri-axis accelerometer with response to dc similar to the VectorSeissensor manufactured by ION Geophysical Corporation of Houston, Tex.,U.S.A., is suitable for many embodiments of the invention. Since thereis no dc component to the seismic wavelet, the dc response of the motionsensor is used to detect the orientation of the sensor relative togravity. One axis of the sensor is designed to be in the knownorientation of the streamer axis. Since the streamer axis orientation isknown and the gravity vector is measured, the orientation of the sensor,and thus the arriving sensed seismic wavelet, can be electronicallyrotated relative to gravity so that up-going seismic wavelets can beaccepted and down-going seismic wavelets rejected.

Any sensors that detect motion can be used. The sensors can be anymotion sensors responsive to position, velocity, or acceleration. Forinstance, a gimbaled first geophone, as described by Tenghamn et al. inU.S. Pat. No. 7,239,577, can be combined with a second geophone,packaged so that it has little or no response to an acoustic wave andthe same response to streamer motion, to achieve the desired result.Piezoelectric accelerometers can be used, as long as they have adequatesensor performance.

If the sensor cannot determine its own orientation, separate orientationsensors can be included in the sensor systems. Alternatively, mechanicalmeans—such as a gimbal system—can be used to fix the sensors in a knownorientation. Winged devices attached to the streamer, sometimes referredto as birds, can also be used to force the sensor into a desiredorientation.

The invention is not meant to be limited to use in towed marinestreamers. The techniques described can also be used in other platforms,such as ocean-bottom cables and autonomous node systems. Additionally,the sensor systems described can be employed for the gathering ofseismic data individually; or they can be strung together and usedcollectively, their data combining to reduce the impact of local flowpatterns.

What is claimed is:
 1. An underwater seismic system comprising: a firstmotion sensor usable on an underwater platform and having a firstresponse; a second motion sensor disposed proximate to the first motionsensor and having a second response; wherein the first and secondresponses are similar for platform motion and different for acousticwaves.
 2. An underwater seismic system as in claim 1 further comprisinga divider disposed between the first and second motion sensors.
 3. Anunderwater seismic system as in claim 1 comprising: a first regionenclosed in a first rigid, perforated housing covered with anacoustically transparent skin, wherein the first motion sensor residesin the first region with a first acoustic impedance; and a second regionenclosed in a second rigid housing having a second acoustic impedance,wherein the second motion sensor resides in the second region.
 4. Anunderwater seismic system as in claim 3 further comprising a fluidfilling the first region and having an acoustic impedance equal to thatof seawater and air filling the second region.
 5. An underwater seismicsystem as in claim 1 further comprising: a rigid body coupled to theunderwater platform and subject to platform motion; a plurality of thefirst motion sensors coupled to the rigid body and a plurality of thesecond motion sensors coupled to the rigid body.
 6. An underwaterseismic system as in claim 5 wherein the rigid body has a tubular shapewith an inner side and an outer side and wherein the plurality of firstmotion sensors are connected to the outer side and the plurality ofsecond motion sensors are connected to the inner side.
 7. An underwaterseismic system as in claim 5 comprising: a first test mass and a secondtest mass; wherein the rigid body has a tubular shape dividing theseismic system into an external region receiving the first test mass andan internal region receiving the second test mass; and wherein theplurality of first motion sensors couples the first test mass to therigid body and the plurality of second motion sensors couples the secondtest mass to the rigid body.
 8. An underwater seismic system as in claim5 comprising: a first test mass encircling the rigid body; a second testmass encircled by the rigid body; wherein the plurality of first motionsensors couples the first test mass to the rigid body and the pluralityof second motion sensors couples the second test mass to the rigid body.9. An underwater seismic system as in claim 8 wherein the first testmass, the rigid body, and the second test mass are coaxially arranged.10. An underwater seismic system as in claim 1 further comprising: afirst rigid housing rigidly coupled to the first motion sensor; a secondrigid housing rigidly coupled to the second motion sensor; a dividerdisposed between the first and second rigid housings to acousticallyisolate the first and second motion sensors in to separate first andsecond regions; wherein the first and second housings have differentacoustic cross sections to incident acoustic waves.
 11. An underwaterseismic system as in claim 10 comprising: an exterior surrounding thefirst and second regions; a first coupling between the first rigidhousing and the exterior; and a second coupling between the second rigidhousing and the exterior.
 12. An underwater seismic system as in claim 1further comprising: a first rigid housing rigidly coupled to the firstmotion sensor; a second rigid housing for the second motion sensor; atest mass inside the second rigid housing and non-rigidly coupled to thesecond rigid housing by the second motion sensor; wherein the first andsecond motion sensors respond in phase to acoustic waves and opposite inphase to platform motion.
 13. An underwater seismic system as in claim 1wherein the first motion sensor produces a first sensor signal and thesecond motion sensor produces a second sensor signal, the underwaterseismic system further comprising means for combining the first sensorsignal and the second sensor signal to attenuate noise due to platformmotion and produce a response due to acoustic waves.
 14. An underwaterseismic system as in claim 13 wherein the hydrophone and the first andsecond motion sensors are in close proximity.
 15. An underwater seismicsystem as in claim 1 further comprising a hydrophone producing ahydrophone signal and p-z summation means for combining thereduced-noise response and the hydrophone signal to produce a seismicresponse signal.
 16. An underwater seismic system as in claim 1 whereinthe first motion sensor has a first impedance and the second motionsensor has a second acoustic impedance and wherein the first acousticimpedance is equal to the acoustic impedance of seawater.
 17. Anunderwater seismic system as in claim 1 comprising a sensor cableserving as the underwater platform, wherein a plurality of the first andsecond motion sensors are disposed along the sensor cable at spacedapart locations.
 18. An underwater seismic system as in claim 1comprising a cable-positioning device serving as the underwaterplatform, wherein the first and second motion sensors are mounted in thecable-positioning device.
 19. An underwater seismic system comprising: afirst motion sensor usable on an underwater platform and having a firstacoustic impedance to produce a first sensor signal representingplatform motion and particle motion due to acoustic waves; a secondmotion sensor disposed proximate to the first motion sensor and having asecond acoustic impedance to produce a second sensor signal representingplatform motion and representing attenuated particle motion due toacoustic waves; means for combining the first sensor signal and thesecond sensor signal to attenuate noise due to platform motion andproduce a response to particle motion due to acoustic waves.
 20. Anunderwater seismic system comprising: a first motion sensor usable on anunderwater platform; a second motion sensor disposed proximate to thefirst motion; an acoustic shield arranged to shield only the secondmotion sensor from acoustic wave particle motion.